1. Field of the Invention
The invention relates to means to identify, monitor and take action upon potential delamination of ceramic coatings used as a thermal barrier for turbine components.
2. Background Information
Cobalt or nickel based superalloys of, for example, IN738, or ECV768 are used for making blades, vanes and other components of gas turbines. These turbines can operate at temperatures in the range of 1000 C. to 1600 C. and are generally protected by a series of protective coatings. The coatings usually comprise layers of metallic base coats, service formed aluminum oxide layers and a final ceramic thermal barrier coating (“TBC”). The TBC is usually made of yttria, ceria or scandia stabilized zirconia, as taught, for example, by U.S. Pat. Nos. 5,180,285; 5,562,998; 5,683,825 and 5,716,720 (Lau, Strangman, Bruce et al., and Murphy, respectively). Long term exposure of these ceramic coatings to the hostile, high temperature, abrasive environment in which such turbines operate can cause phase destabilization, sintering, microcracking, delamination and ultimately spallation within the coating layers, exposing the superalloy component to degradation or failure and requiring expensive repairs.
Many attempts have been made to non-destructively test such coated superalloy metal surfaces for non-obvious, subcoating degradation. U.S. Pat. No. 4,647,220 (Adams et al.) teach a system to detect corrosion and stress corrosion cracking of painted metal structures, utilizing infrared thermographic techniques to detect temperature differentials caused by the difference in thermal conductives between corroded metal and uncorroded metal. A scanner can be used to produce a television-compatible, video output signal of the thermophysical characteristics it is viewing. This system is used primarily on stationary military aircraft. U.S. Pat. No. 5,294,198 (Schlagheck) teaches a system to determine defects in commercial products by obtaining an infrared image of the product while it is being stimulated. An infrared television monitor supplies a signal to a color monitor where hot or cold temperature regions appear as red or blue respectively. Defects can be determined by an inspector or a computer. This system can also be incorporated into production lines, and eliminates prolonged vibration and/or temperature cycling as tests of commercial products.
In U.S. Pat. No. 5,272,340 (Anbar) teaches an infrared imaging system which simultaneously generates temperature, emissivity and fluorescence, for use in clinical diagnosis and management of skin disorders, to determine true skin temperature as a tool in the treatment of malignancies, burns and the like. U.S. Pat. No. 5,608,845 (Ohtsuka et al.) relates to predicting the remaining lifetime, by parts degradation analysis, of, for example, carbon seals, electrically operated values, control rod drivers, and the like, in locations such as electric power plants. This appears to be accomplished by establishing a series of lifetimes based on experimental aging degradation data.
In U.S. Pat. No. 5,552,711 (Deegan et al.), probable turbine blade failure is determined by measuring specific ions emitted by hot spots. The invention relates to electromagnetic energy radiated by ions that are created as combustion gas erodes and ionizes materials in these hot spots using spectral detectors looking for characteristic ions. However, this system requires failure to occur, for example by melting of components and detection of ions. Turbine blade temperature monitors are taught by U.S. Pat. No. 5,306,088 and 5,832,421 (Zoerner and Santoso et al., respectively). Zoerner requires an actual fiber-optical cable actually disposed inside a turbine component. Santoso et al. require measurement of pressure and temperature at locations other than the blades and then simulating blade temperature values using a water stream cycle analysis program and then training an artificial network so that it can learn to recognize a failure by estimating blade temperature.
In U.S. Pat. No. 4,764,025 (Jensen), a temperature detection pyrometer is used to determine turbine blade temperature from radiation reflected and emitted from the blade. The system substantially reduces the effect of reflected radiation from flame or hot carbon particles. Detected radiation is divided into two channels and the output of one of the detectors is weighted relative to the other. The difference between the factored output from one detector and the output from the other detector is provided to a difference amplifier to provide a signal directly related to the temperature of the turbine blade.
There is still special need, however, to be able to sense potential failure situations for complex, moving turbine components having ceramic coating layers, by utilizing a very fast sensor system. This would require measuring relative spatial/time radiance using an expert system, and some sort of a degradation model that will generate advisory information and actively avert failure. This system must identify very small hot spots on low-IR emittance ceramic surfaces, detect spalling and debond areas, measure their growth, and forecast and prevent failure.